Link libdeflate for OpenEXR 3.2+ (cmake)

[Razzaline]

This just adds a line to the main CMake file, fixing #1248 for now (not compiling in Linux because of libdeflate not being explicitly linked). If there's a better fix, I don't mind others suggesting something, or adding to or changing it.

[manongjohn]

The build failed as I had expected since currently libdeflate is not a required dependency and therefore not loaded on current Ubuntu builds.

/usr/bin/ld: cannot find -ldeflate
clang: error: linker command failed with exit code 1 (use -v to see invocation)

[Razzaline]

Oh, that makes sense >.< I just looked into it and it does appear that Ubuntu and a lot of other distributions don't have OpenEXR 3.2+ in their package repositories. I don't really know how to proceed or check for that, though... Maybe with a CMake module checking the OpenEXR version? Technically, OpenCV can be compiled without OpenEXR, but every distro that I looked at seems to require OpenEXR with OpenCV.

[manongjohn]

As part of T2D builds, I locally compile OpenCV to adhere to an LGPL compliant license. It's possible I have exluded OpenEXR on purpose which explains why it isnt looking for it or its dependencies (i.e. libdeflate).

EDIT: I confirmed my build of OpenCV excludes OpenEXR.

[Razzaline]

Okay, that makes sense. After you mentioned that OpenEXR wasn't included, I checked and noticed that in the CI build scripts.

Sorry about the merge commit there >.< I hadn't realised it wouldn't ask me for confirmation when I told it to sync. But I've added a hopefully slightly better solution? It checks for the OpenEXR version and if it doesn't find it I think it should just ignore it. I don't have a lot of experience with CMake so I'm not entirely sure. Otherwise, it does seem to be linking libdeflate properly for me.

[Razzaline]

I can probably add the same logic for Windows and Mac OS, in the unlikely event that someone's build environment happens to have OpenEXR 3.2+ on an OpenEXR-enabled build, although I guess that might happen eventually anyway so it's good to have the checks in place.

[Razzaline]

For now I just added the Apple build environment stuff. Adding the check for MSVC might involve creating a FindOpenEXR.cmake (there are no prebuilt binary releases of OpenEXR for Windows) so I thought I should leave it for later.

[Razzaline]

I'm planning to finish this soon if possible (and was hoping to before the 1.4 release), but I've been stuck trying to figure out the Windows part of it (hence running a build in a virtual machine and encountering #1300). I've made a few additional changes I haven't pushed yet, but if all else fails I could just push what I have with the Windows parts commented out (though they are written in a way that under normal circumstances CMake should just skip over that).